In an electrical communication system, it is sometimes advantageous to transmit information signals (e.g., video, audio, data) over a pair of wires (hereinafter “wire-pair” or “differential pair”) rather than a single wire, wherein the transmitted signal comprises the voltage difference between the wires without regard to the absolute voltages present. Each wire in a wire-pair is susceptible to picking up electrical noise from sources such as lightning, automobile spark plugs, and radio stations, to name but a few. Because this type of noise is common to both wires within a pair, the differential signal is typically not disturbed. This is a fundamental reason for having closely spaced differential pairs.
Of greater concern, however, is the electrical noise that is picked up from nearby wires or pairs of wires that may extend in the same general direction for some distances and not cancel differentially on the victim pair. This is referred to as “crosstalk.” Particularly, in a communication system involving networked computers, channels are formed by cascading plugs, jacks and cable segments. In such channels, a modular plug (see, e.g., plug 10 and entering cable 20 in FIG. 1) often mates with a modular jack, and the proximities and routings of the electrical wires (conductors) and contacting structures within the jack and/or plug also can produce capacitive as well as inductive couplings that generate near-end crosstalk (NEXT) (i.e., the crosstalk measured at an input location corresponding to a source at the same location) as well as far-end crosstalk (FEXT) (i.e., the crosstalk measured at the output location corresponding to a source at the input location). Such crosstalks can occur from closely-positioned wires.
Communication system infrastructure using the “Ethernet” standard is based on data being transmitted differentially on up to four twisted-pair transmission lines (designated as Pair 1 through Pair 4) grouped together within a common cable jacket. As described above, the transmission lines are connected with physical connectors. In order to maintain backwards-compatibility with legacy systems, the physical requirements for the connectors have been fixed by industry standards (see, e.g., TIA/EIA 568-B.2-1, FIG. 6-2 D.25). These requirements are not necessarily optimum for high speed data transmission.
For historical reasons, the four twisted pair transmission lines are arranged at the connectors such that the two wires that make up Pair 3 split apart and connect on alternate sides of Pair 1. The remaining Pairs 2 and 4 lie on either side of the split pair combination (see conductors 20a–20h and blades 30a–30h in FIG. 2). The electrical properties, in particular the degree of crosstalk between the pairs, are impacted by this physical layout.
To maintain the compatibility of components between different vendors, a “Nominal” plug response was defined and accepted as an industry standard (see, e.g., TIA/EIA 568-B.2-1). A range of allowable variation was also defined and accepted, which enables mating “jacks” to complete the connection of the twisted pair cables with resultant levels of crosstalk between the twisted-pair transmission lines reduced to some required value. This process of reducing the resultant crosstalk levels has been commonly termed “compensation” and is essentially the intentional addition of signals that sum up to be of equal magnitude but opposite sign to that of the original offending crosstalk.
The accepted levels of crosstalk for a “Nominal Plug,” in particular for the Pair 1-Pair 3 combination, are fairly restrictive. As a result, little change has been made to the plug structure to improve overall system performance, although improvements in areas such as manufacturability, cost, and variability have been made. Until recently, because of the restrictive predefined crosstalk levels for the plugs, improvements in system performance have been mainly a result of compensation techniques in the jack receptacle.
As data transmission rates have increased, a variety of system performance requirements have evolved. While the data sent through the system is still sent via the four twisted-pair transmission lines, the levels of permissible crosstalk between the pairs (i.e. interference) in a working system has decreased in spite of advances in signal processing techniques and coding schemes. In previous requirements, the levels of interference have been defined for signals within a single four-pair cable. This was because the absolute levels of interference from pairs in other physically close cables were negligible as compared to levels from other pairs internal to a single cable. However, with the new standards and high data rates that are evolving, this is no longer true. The interference received on a four-pair cable as a result of transmission on other cables or connectors has been termed “Alien Crosstalk”.
Since the newly defined alien crosstalk can be generated from any unrelated data transmission, it can be difficult to use current signal processing techniques to calculate and subtract away their effects within a four-pair cable connection (referred to as a “port”). As a result, the absolute levels of alien crosstalk are lower than those allowed to exist from pairs within a cable bundle because no digital signal processing (DSP) correction is applied.
Another issue with alien crosstalk results from the fact that alien crosstalk levels vary based on a number of random factors such as how adjacent cables are bundled together, the physical proximity of the plugs and jacks within a given system, the number of cables adjacent to each other, and the like. All of these factors cannot be known a priori to the design of the compensating network. As a result of these factors, the degree to which alien crosstalk can be “corrected” for is limited, and alien crosstalk can ultimately dominate the final system performance levels.
Alien crosstalk received within a cable pair is due to that cable pair being positioned within the electromagnetic fields generated by other cables or connectors. The inherent structure of these fields determines the strength of the crosstalk signals that are ultimately induced. As a result, increasing the physical separation between the conductor pairs usually results in decreased levels of crosstalk due to the inverse relationship between field strength and distance from the source.
The field structure of a transmission line is determined mainly by its cross-sectional structure. For a two-conductor transmission line, increasing the separation between conductors generally causes the field patterns to become more spread out, which can result in increased levels of crosstalk for a fixed physical separation between cables.
As previously explained, the physical structure of the Nominal Plug is limited by the constraints placed on the internal crosstalk parameters. This physical structure does not maintain symmetry between the four pairs internal to a cable. As a result, a differential signal transmitted on Pair 3 will couple different absolute voltage levels onto Pair 2 and Pair 4. The differential signal on Pair 3 is said to couple a “common” voltage onto Pair 2 and Pair 4. However, the two “common mode” signals coupled to the outer pairs results in a new differential signal that uses Pair 2 as a single effective conductor and Pair 4 as the other; it is effectively another transmission line within the cable bundle. However, since the two pairs are physically separated by more distance than a single twisted pair, the resulting field structure will be less confined and therefore can cause more alien crosstalk onto nearby cable pairs than the direct crosstalk from the internal Pair 3 signal.
Given the number of conductor pairs within a cable, the number of cables in a system, the number of connectors in a system, etc., it is clear that numerous mechanisms (both direct and indirect) for alien crosstalk can exist, with the previous example being a dominant mechanism in at least some cable systems. One possible solution to the alien crosstalk problem is the use of shielded transmission line cables and connectors, commonly referred to as “foil twisted pairs” (FTP). Although shielding can be an effective solution to alien crosstalk, it is not consistent with an unshielded twisted pair installation base and is typically more expensive to manufacture and install.